Recent work, using cultured skin fibroblasts, has shown that the defect in familial hypercholesterolemia lies in faulty feedback regulation of cholesterol biosynthesis. Homozygotes have extremely high serum cholesterol levels and short life expectancy. Heterozygotes have increased coronary artery disease and a Type II lipoprotein pattern with elevated levels of low density lipoproteins (LDL). By use of lipoproteins labeled with 125I in the apoprotein and 3H cholesterol in the lipid moiety, we have shown that cholesterol uptake by cultured fibroblasts involves an initial interaction with specific receptors for HDL and LDL at the cell membrane, followed by transfer of cholesterol into the cell. Goldstein and Brown have shown that in hypercholesterolemic fibroblasts feedback regulation fails because uptake of LDL cholesterol is blocked. These cells do not bind 125I labeled LDL. In the continuing studies proposed here, the mechanism of uptake of lipoprotein-bound cholesterol by cell lines cultured from normal and hypercholesterolemic individuals will be investigated. The nature and specificity of the membrane receptors for the different classes of lipoproteins will be elucidated and their relationship to the regulation of cell cholesterol content determined. When the characteristics of the various components have been defined, the investigation will focus on the isolation and purification of the LDL receptor in order to determine the molecular nature of the cellular lesion associated with hypercholesterolemia. BIBLIOGRAPHIC REFERENCES: J.M. Bailey, T.J. Allen, J. Butler, J.D. Wu, Defective Regulation of Cholesterol Biosynthesis in Tumor Virus Transformed and Hypercholesterolemic Skin Fibroblasts, VIII Annual Biochem. Winter Symposium, 136, (1976), Abstr. Lipoprotein Uptake by Cultured Cells, J.D. Wu, E.J. Butler, and J.M. Bailey, Am. Soc. Microbiol. I 43 (1976) Abstr.